Night shift and cancer Q&A

The increased risk of breast cancer in female night shift workers is 'modest'

29 mar 2009--Denmark has begun compensating “dozens” of women who developed breast cancer after working night shifts, multiple news sources have reported. The BBC said the Danish government’s decision is based on a report from WHO’s International Agency for Research on Cancer (IARC), which concluded that working nightshifts could increase women’s risk of breast cancer.

The report from the IARC has not yet been published. A summary of the report says that most of the epidemiological studies it looked at "found a modestly
increased risk of breast cancer in long-term employees compared with those who are not engaged in shift work at night".

However, the summary also said these studies have some limitations, including the possibility that factors other than shift work may have affected the results. The UK’s Health and Safety Executive (HSE) has commissioned its own report on the health impact of night-shift work, including its effects on breast cancer risk. This research is due to be published in 2011. This report will help UK policy makers to decide whether to make changes to recommended work practices.

In the interim, Cancer Research suggests that the advice for shift workers is the same as for other women: to remain breast aware, to visit their GPs if they notice anything unusual about their breasts, and to take up invitations for breast screening.

Why is the Danish government making this payout?

The Danish government’s decision to compensate the women is based on a report by the WHO’s International Agency for Research on Cancer (IARC). The report originated from a specially commissioned expert working group. The group met in October 2007, when they concluded that “shift work that involves circadian disruption [working at night time when a person would normally be sleeping] is probably carcinogenic to humans”.

What is the evidence that working at night increases the risk of breast cancer?

The conclusion of the working group was based on “limited evidence in humans” that shift-work involving night work is carcinogenic. It also took into account the “sufficient evidence” from animal experiments that exposure to light during the daily dark period (known as biological night) is carcinogenic.

The findings of the working group were summarised in the Lancet Oncology, but the full report has not yet been published. The summary reports that six out of eight cohort studies it looked at, found a “modestly increased” risk of breast cancer among long-term employees who worked night shifts, compared to those who did not. They say that these studies do have some limitations, including the possibility that factors other than shift work may have affected the results. Additionally, the studies use different definitions of what shift work is, and several of the studies focus on single professions only (mainly nurses or flight attendants).

The working group also looked at animal experiments. They describe studies in rodents, which looked at the effect of disrupting the animals’ normal light-dark cycle on tumour development. The summary reported that more than 20 studies in rodents looked at the effects of constant light, dim light at night, simulated jet lag, or “circadian timing of carcinogens”. Most studies found an increase in the number of tumours.

It also reported that a similar number of studies in rodents looked at the effect of reducing the normal nighttime production of the hormone melatonin in rodents by removing the gland that makes this hormone. Most of these studies also reported an increased number and growth of tumours.

How great is the increase in risk?

The summary of the IARC findings does not provide an overall estimate of how much women’s risk is increased or for how long a woman has to work nights before her risk is increased.

One of the cohort studies that it reviewed was carried out in over 70,000 female nurses in the US, and followed them up for 10 years. This study found that about 42 in every 1,000 nurses who worked for 30 years or more on rotating night shifts developed breast cancer compared with about 29 in every 1,000 nurses who never worked night shifts.

This represents a 36% increase in the risk of breast cancer for women who worked for 30 years or more on rotating night shifts. About 32 out of every 1,000 nurses who worked on night shifts for less than 30 years developed breast cancer, and this represented an 8% increase in risk of breast cancer compared with women who never worked night shifts.

I read that only women exposed to asbestos are at greater peril. Is this true?

Various newspapers have compared the effects of night-shift work to that of “anabolic steroids, ultraviolet radiation and diesel engine exhaust”. The Mirror reported that “women who work nights are at such serious risk of cancer that only those exposed to substances such as asbestos are in greater peril”.

These comparisons appear to be based on the grading that the IARC have given to shift work. Based on the evidence available,the IARC grades potential cancer-causing hazards, and then groups them accordingly. There are five groups:

• Group 1: the agent is carcinogenic to humans.
• Group 2A: the agent is probably carcinogenic to humans.
• Group 2B: the agent is possibly carcinogenic to humans.
• Group 3: the agent’s carcinogenicity to humans is not classifiable.
• Group 4: the agent is probably not carcinogenic to humans.

Shift work at night has been put in Group 2A – with asbestos rated as a Group 1 agent. However, it is important to note that this grading system is based on how much evidence there is to support the view that the agent in question has a cancer-causing (carcinogenic) effect.

Group 1 means that there is sufficient evidence to conclude that a factor causes cancer in humans, while Group 2A means that there is limited evidence that the factor causes cancer in humans, but sufficient evidence that it can cause cancer in experimental animals. Therefore, these groupings do not give a measure of how much a factor increases cancer risk.

How might working at night increase cancer risk?

It is not clear exactly how working at night might increase risk of cancer. There is a theory that disruption of the circadian system and the hormone melatonin are involved. Working at night is known to disrupt our circadian system, which regulates how we respond to night and day. This system affects how active we are, which hormones are produced, and which genes are switched on and off. Some of the genes affected by the circadian system can affect tumour growth, while the hormone melatonin, which is normally produced at night, affects immune system function.

What happens now?

The UK’s Health and Safety Executive (HSE) has commissioned its own report on the health impact of night-shift work, including its effects on breast cancer risk. This research is being carried out at the University of Oxford Cancer Epidemiology Unit, and the report is due to be published in 2011.

This report will help UK policy makers to decide whether to make changes to recommended work practices. In the interim, Cancer Research suggests that the advice for shift workers is the same as for other women: to remain breast aware, to visit their GPs if they notice anything unusual about their breasts, and to take up invitations for breast screening.